To add the Send Email to user action to the workflow, click on the "Send Email to user" icon.
To set the parameters, click on the node.
The settings window will appear.
Insert the name of the node.
Select the email template.
You can add an attachment.
Select the users to be sent.
When everything is set, click on the "Confirm" button.
To test the email template, click on the node.
Then "Send preview" button.